1. Field of the Invention
The present invention relates to an apparatus for driving a liquid crystal display device, and more particularly, to an apparatus and method for driving a liquid crystal display device, wherein the response speed of a liquid crystal can be increased only when displaying a moving image.
2. Related Art
Liquid crystal display devices adjust light transmittance of liquid crystal cells according to a video signal so as to display an image. An active matrix type liquid crystal display device has a switching element for every liquid crystal cell and is suitable for the display of a moving image. A thin film transistor (TFT) may be used as the switching element in the active matrix type liquid crystal display device.
However, a liquid crystal display device may have a relatively slow response speed due to characteristics such as the inherent viscosity and elasticity of a liquid crystal, as can be seen from the following Equations 1 and 2.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          Va                  2                                -                                  V                  F                  2                                                                                                      [                  Equaiton          ⁢                                          ⁢          1                ]            In equation 1, τr is a rising time when a voltage is applied to the liquid crystal. Va is the applied voltage, VF is a Freederick transition voltage at which liquid crystal molecules start to be inclined, d is a liquid crystal cell gap, and γ is the rotational viscosity of the liquid crystal molecules.
                              τ          F                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  Equation          ⁢                                          ⁢          2                ]            In equation 2, τF is a falling time when the liquid crystal is returned to its original position owing to an elastic restoration force after the voltage applied to the liquid crystal is turned off, and K is the inherent elastic modulus of the liquid crystal.
In a twisted nematic (TN) mode liquid crystal display device, although the response speed of the liquid crystal may be different according to the physical properties and cell gap of the liquid crystal, rising times may be about 20 ms to about 80 ms and falling times may be about 20 ms to 30 ms. Because liquid crystal response speeds may be longer than one frame period (e.g., 16.67 ms according to the National Television Standards Committee (NTSC)) of a moving image, the response of the liquid crystal proceeds to the next frame before a voltage being charged on the liquid crystal reaches a desired level. This results in motion blurring in which an afterimage is left in the eyeplane.
FIG. 1 is a waveform diagram illustrating a data-dependent brightness variation in a related art liquid crystal display device. Some related art liquid crystal display devices cannot express a desired color and brightness for display of a moving image. In FIG. 1, when data VD is changed from one level to another level, the corresponding display brightness level BL is unable to reach a desired value due to a slow response speed of the liquid crystal display device. As a result, motion blurring occurs in the moving image, thereby causing degradation in contrast ratio and in display quality.
In order to solve the low response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/09967 have proposed a method for modulating data according to a variation therein using a look-up table (hereinafter, referred to as a ‘high-speed driving method’). This high-speed driving method is adapted to modulate data on the basis of a principle as shown in FIG. 2.
With reference to FIG. 2, the related art high-speed driving method includes modulating input data VD and applying the modulated data MVD to a liquid crystal cell to obtain a desired brightness level MBL. In this high-speed driving method, in order to obtain the desired brightness level corresponding to the luminance of the input data in one frame period, the response of a liquid crystal is rapidly accelerated by increasing |Va2−VF2| in Equation 1 on the basis of a variation in the input data.
In detail, the related art high-speed driving method generates a modulated data signal Mdata by comparing source data RGB of a previous frame Fn−1 with source data RGB of a current frame Fn, as shown in FIG. 3.
In FIG. 3, a related art high-speed driving apparatus includes a frame memory 43 storing the source data RGB of the current frame Fn input from a data bus line 42, and a look-up table 44 outputting the modulated data signal Mdata by comparing the source data RGB of the previous frame Fn−1 with the source data RGB of the current frame Fn.
The frame memory 43 stores the source data RGB of the current frame Fn for one frame period, and supplies the stored source data RGB to the look-up table 44. The look-up table 44 provides a modulated data signal for accelerating the response speed of the liquid crystal on the basis of variation of the previous frame Fn−1 and the current frame Fn. Thus, the look-up table 44 compares the source data RGB of the current frame Fn input from the data bus line 42 with the source data RGB of the previous frame Fn−1 input from the frame memory 43 and selects modulated data Mdata corresponding to the comparison result.
Accordingly, the liquid crystal display device using the related art high-speed driving method can compensate for a slow response of a liquid crystal by modulation of a data value to relax motion blurring in a moving image, so as to display an image with a desired color and brightness.
However, the liquid crystal display device using the related art high-speed driving method improves picture quality by relaxing motion blurring for the display of a moving image but increases power consumption by accelerating the response speed of the liquid crystal even in the case of a general image or a still image (text) in addition to the moving image.